Avoiding long-running cracks in natural gas pipelines

Despite the world’s ambition to abandon fossil fuels and to move to renewable energy sources, natural gas will remain an important energy source in the coming decades. In fact, the worldwide demand for natural gas is still increasing, forcing oil and gas majors to consider gas reserves located in more hostile and remote regions. Consequently, new pipeline projects are continuously pushing the limits in terms of higher design pressures and lower operating temperatures – and thus requiring high-strength steel grades with excellent low temperature toughness.

FRACTURE CONTROL

One of the most important aspects of pipeline design is fracture control – which basically means the avoidance of long-running cracks – as such events would involve huge economic losses and dramatic environmental damage. One can distinguish between crack initiation and crack propagation control but, in the case of gas pipelines, the requirements for crack initiation control are, in general, overruled by the more stringent requirements for crack propagation control. To avoid long-running cracks, gas decompression should occur faster than crack propagation. This can be achieved by fulfilling two requirements, as specified in different international standards, such as API 5L and ISO 3183: first, the ductileto-brittle transition temperature (DBTT) of the pipe material should be lower than the temperature at which the pipeline is operated in order to guarantee ductile crack propagation. The DBTT is defined as the temperature at which the fracture surface of a Battelle Drop Weight Tear Test (BDWTT) sample shows 85% shear area. Second, sufficient energy should be absorbed during crack propagation to arrest a running crack. The minimum required energy is most often determined by means of the Battelle Two Curve Method and is expressed in terms of upper-shelf Charpy impact energy.

TOOL FOR AUTOMATED FRACTURE SURFACE INSPECTION

The inspection of the BDWTT fracture surface of heavy-gauge, modern pipeline steels – for instance, X70 and X80 – can be complicated by the presence of specific features such as splits. Consequently, inspection results can be operatordependent. Therefore, a round robin was organised between OCAS and the quality labs of the ArcelorMittal plants concerned to

define a common practice for BDWTT testing and BDWTT fracture surface evaluation. Furthermore, OCAS started developing a software tool for automated inspection, which will make use of machine learning techniques. Such a tool will significantly reduce inspection times and will eliminate operator dependence.

INSTRUMENTED BDWTT EQUIPMENT

Due to their large crack initiation toughness, heavy-gauge high-strength pipeline steels often show the problem of inverse fracture during BDWTT testing, meaning that a ductile propagating crack turns into a brittle propagating crack towards the end of the test. Although the occurrence of inverse fracture is accepted by the latest version of API 5L, it should be avoided as it leads to underrating the material’s toughness. Several research groups have proposed alternative BDWTT notch types, aiming at lowering the crack initiation energy and thus reducing the risk of having inverse fracture. OCAS launched several experimental studies to assess the effectiveness of those notch types. To support these activities, OCAS also invested in instrumented BDWTT equipment, which allows us to evaluate how much energy is absorbed during the test and to differentiate between the crack initiation and the crack propagation energy. Furthermore, it provides valuable information for evaluating the effect of inverse fracture and the performance of different BDWTT notch types. Because of the problem of inverse fracture, standardisation bodies are discussing alternative fracture toughness requirements, one of them being BDWTT crack propagation energy, which requires instrumented BDWTT equipment. Thus, by investing in this equipment, OCAS anticipated future changes in international standards. Our current BDWTT equipment has a maximum capacity of 60 kJ. The maximum 2.5 m drop height and 2450 kg hammer weight can be varied, making it a flexible tool for research purposes.

DYNAMIC TEAR TEST: MEDIUM SCALE CRACK PROPAGATION TEST

In addition to standardised (dynamic) fracture toughness tests, OCAS studies the crack arrest capabilities of pipeline grades by means of Dynamic Tear Testing (DTT), which presents several advantages in comparison to the BDWTT and Charpy impact testing: i) the sample is mainly

Although dynamic tear testing is non-standardised, it is a valuable research tool that provides insights that cannot be obtained from standard tests, such as BDWTT and Charpy impact tests.

Steven Cooreman

loaded in tension, which reduces the risk of inverse fracture and is closer to the loading conditions present during crack propagation in a gas pipeline, and ii) as the sample is considerably larger, the crack propagates over a longer distance. Although it concerns a non-standardised test, the DTT set-up is a valuable research tool. It is instrumented to capture force and displacement and thus provides information on the crack initiation and propagation energy. In the framework of an OCAS sponsored PhD thesis, a high-speed Digital Image Correlation system was used to monitor how the crack develops and propagates and to evaluate the deformation in the vicinity of the crack tip. By doing so, even more data could be extracted – for example, the crack tip opening angle (CTOA), an alternative parameter for expressing the resistance against crack propagation. Moreover, the data obtained in this way revealed that a region of stable crack propagation exists. The PhD student is also developing a system for cooling the samples on-line, which will allow us to study the fracture behaviour in the ductileto-brittle transition region.